US President Barack Obama (R) and British Prime Minister Theresa May hold a press conference on the sidelines of the G20 summit in Hangzhou on September 4, 2016. (AFP Photo)

President Barack Obama has said the US and Russia remain deeply divided over the Syria crisis, adding it is difficult to “get to the next phase.”

"We have grave differences with the Russians in terms of both the parties we support but also the process that is required to bring about peace in Syria," Obama said in a joint press conference with UK Prime Minister Theresa May in Hangzhou, China, on the sidelines of the Group of 20 economic summit.

“We’re not there yet,” he said. “I think it’s premature for us to say there’s a clear path forward, but there’s the possibility at least for us to make some progress.”

His remarks come as US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov are negotiating a deal to boost military cooperation between the two countries in the fight against Daesh in Syria.

According to a senior US State Department official, Washington is close to a deal with Russia, but some issues remain to be resolved.

The inability to achieve a deal will bring another missed deadline for the UN special envoy for Syria, Staffan de Mistura, to get the Syrian government and "moderate" militants back to the negotiating table.

During the press conference, Obama said the US and the UK must cooperate to ensure that their ongoing trade links do not unravel after Britain leaves the European Union.

“I’ve committed to Theresa that we will consult closely with her as she and her government move forward on Brexit negotiations to make sure we don’t see adverse effects in our trading and commercial relationship.”

Trade “is not going to stop,” Obama said. "We're going to do everything we can to make sure that the consequences of the [Brexit] decision don't end up unraveling what is already a very strong and robust economic relationship."

“But first things first. The first task is figuring out what Brexit means with respect to Europe.”

The UK prime minister said she had used her first meeting with Obama to discuss the Brexit process and what it meant to the UK's relationships with the US and European countries.

"The UK has always been a strong partner for the US and that will remain to be the case. We have a thriving economic relationship,” May said.

Nearly 52 percent of British voters opted to leave the EU in a referendum on June 23, in hopes of taking back control over their borders and having more economic freedom.